In a wireless communication system, a radio access network (RAN) usually comprises at least one mobile switching center (MSC), base station controller (BSC), and base transceiver system (BTS). Each BTS may radiate to define wireless coverage areas that serve wireless communication devices (WCDs) such as cell phones.
Many users of wireless communication systems subscribe to voicemail services. When a callee does not answer an incoming call, a voicemail service allows the caller to leave a message for the callee. The callee may not answer the incoming call for various reasons. The callee's WCD may be turned off, or it may be out of wireless coverage. Alternatively, the WCD may on and within a wireless coverage area, but the callee may not answer the incoming call because the callee is not aware of an incoming call indication (such as a ring tone), or chooses to ignore the incoming call indication (e.g., the callee is on another call).
Regardless of the reason, when a callee who subscribes to a voicemail service misses a call, the caller will be directed to a voicemail storage system. Typically, after listening to a short prompt, the caller has the opportunity to leave a message for the callee on the voicemail storage system. If the caller leaves a voicemail message, the callee may later retrieve and listen to the message.
Most wireless communication systems provide a notification to the callee's WCD when such a message is left for the callee. This notification may take the form of a message waiting indicator (MWI). The MWI may indicate that there is at least one voicemail message waiting for the callee. Upon receiving an MWI, the callee's WCD may react accordingly, such as emitting a sound, vibrating, or causing a light on the WCD to glow or to flash on and off.
However, in some cases, the MWI may not be properly delivered to the callee's WCD. This can happen for many reasons, including the voicemail system being overloaded, network congestion causing the MWI to be lost, or the MWI being delivered to a RAN other than the RAN serving the WCD. In these cases, the callee's WCD will not be able to immediately notify the callee that there is a voicemail left for the callee. The callee may be notified of the voicemail at some later point, such as when the callee's WCD re-registers with the same or a different RAN, or when another voicemail is left for the callee, and then an associated MWI is successfully delivered to the callee's WCD. However, the callee may have to wait minutes, hours, or even days for this subsequent notification to arrive. In the mean time, the callee may be unaware that there is a potentially urgent voicemail message waiting on the voicemail storage system.